1. Field of the Invention
The invention is generally concerned with a variable speed drive pulley of the kind comprising a hub and, facing one another on this hub, two annular flanges with frustoconical surfaces between which a drive belt can be gripped to transmit a given torque.
It is more particularly concerned with an improvement to this type of pulley whereby the gripping of the belt can be stabilized whatever the rotation speed of the pulley, to minimize wear of the belt.
It is also concerned with a variable speed drive system comprising two pulleys of the known kind described above, namely a driving pulley and a driven pulley, and a drive belt disposed between these two pulleys, the improvement of the invention being more specifically concerned with the driven pulley.
2. Description of the Prior Art
A known type of variable speed drive system comprises two pulleys, namely a driving pulley and a driven pulley, each comprising a hub, and, facing each other, two annular flanges with frustoconical surfaces at least one of which is a movable flange mounted so as to be able to move axially relative to its own hub. A drive belt is disposed between the two pairs of flanges of the two pulleys. The distance between the flanges in each pair thus determines the position of the belt between the pulleys and, consequently, the ratio between the rotation speed of the driving shaft and that of the driven shaft. In each pulley the movable flange has an axial force applied to it by spring means comprising an annular diaphragm one part of which forms a Belleville spring washer. The central part of the diaphragm nearest the hub, divided into radial fingers, is in axial rocking bearing engagement against an annular abutment member fastened to the hub. In conventional practise the diaphragm is prestressed between the annular abutment member and the movable flange in such a way that the belt is gripped between the two flanges, irrespective of its position, so as to be able to transmit a particular torque. Also, there are flyweights constrained to rotate with the diaphragm, generally being fixed to it, in order to deform it in a direction tending to urge the movable flange of the pulley axially towards the other flange, by virtue of the centrifugal forces produced. Because the flyweights are fixed to the diaphragm spring, which deforms, the "lever arm" of the flyweights increases in the case of the driven pulley as the two flanges of the pulley move towards each other, with the result that when the belt is gripped between the outer parts of the flanges the gripping forces are too high relative to the torque to be transmitted. This can result in premature wearing of the belt. A variable speed drive system of this kind is described in French Patent No. 2 522 765. One solution (considered in this document in respect of the driving pulley) would be to articulate the flyweights to the diaphragm spring in such a way that the "lever arm" would vary to only a limited extent as the diaphragm deforms. An arrangement of this kind would be costly, however. An object of the invention is to solve this problem with much simpler means.